Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $231.83   13.768%   $263.7484   $263.75   $263.75   $263.70  
  Purchase #2 $83.32   13.370%   $94.4599   $94.46   $94.45   $94.50  
  Purchase #3 $225.86   4.717%   $236.5138   $236.51   $236.50   $236.50  
  Purchase #4 $27.39   5.368%   $28.8603   $28.86   $28.85   $28.90  
  Purchase #5 $85.29   9.762%   $93.616   $93.62   $93.60   $93.60  
  Purchase #6 $114.62   7.370%   $123.0675   $123.07   $123.05   $123.10  
  Purchase #7 $139.81   8.716%   $151.9958   $152.00   $152.00   $152.00  
  7 purchase totals = $908.12       $992.2617   $992.27   $992.20   $992.30